When a load having a reactance component is driven by use of an AC power supply, a phase difference is generated between voltage and current. Consumed power causes active power and reactive power due to the phase difference. An increase in reactive power does not lead to effective use of the power supply, and thus it is desirable that reactive power is reduced and active power is increased.
A rate of active power relative to power supplied from a power supply is called power factor, and is typically expressed as cos θ. Herein, θ is a phase difference between current and voltage. In order to increase active power as much as possible, a power factor in a power consumption circuit needs to be measured and the circuit needs to be adjusted so as to increase the power factor. In particular, since the power factor directly displays a current drive efficiency, it is desired that the power factor is enhanced for efficient driving and that the power factor is directly measured, in terms of power saving.
In many cases, however, the power factor of power consumed in the circuit is obtained by measuring reactive power. This is found by taking a product of the voltage and current signals mutually shifted by 90° phase. To shift 90° phase, a component such as transformer or integration circuit has been used. However, the method has a problem that calculations of reactive power are complicated during a load variation.
In order to solve the problem, in Patent Literature 1, there are provided a sampling means for sampling a voltage and a current of an AC circuit, a memory means for storing voltage data and current data sampled by the sampling means per sampling point, and a pseudo reactive power calculation means for calculating reactive power from the voltage data and the current data stored in the memory means, in which the memory means stores the voltage data and the current data in the same phase for one cycle, and the pseudo reactive power calculation means cyclically multiplies the voltage data and the current data for a predetermined cycle stored in the memory means by current data at each sampling point and voltage data at a sampling point shifted therefrom by 90° phase, respectively, thereby calculating reactive power based on the average of the multiplication values.